Tires on the ground ...

Suppose we had a sheet of memory metal, or failing that perhaps springs, we keep it flat by sticking plasters of a material that would sublate off in a vacuum. In the vacuum it springs into a sphere and the edges super weld. The weld would degrade in an atmosphere but maybe we could get a good key by using something like bismuth along the seam.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Larry Burford</i>
<br />Use (lasers?) to cut the asteroid in half and carve out two hemisphereical shells which are then welded back together to form a spherical shell, with a hard vacuum trapped inside. Suppose you do this and end up with a shell that is 1 kilometer in diameter and one meter thick? How much iron do you have? <hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">What happens to the material that gets "carved out" of the middle? I'm assuming you're talking about a ship of some kind tracking this asteroid, with a laser capable of cutting a kilometer wide iron asteroid in half, and then carving it out hollow like some gigantic pumpkin. What happens to all the material from the middle? If the shell is still millions of tons, like you said, then hundreds of millions probably got blasted away.

rd

The author never mentioned the left overs (IIRC, it has been a while since I read it). I presume that the volume of material trimmed from the outside would also be larger than the volume of the shell. With lasers capable of carving up an asteroid like this, I guess the parts could be cut, shaped and assembled into still more shells.

One of the problems with speculative science and engineering is that details are sometimes skipped. At least, it is a problem from the point of view of anyone that wants to try to run with the idea. From the author's point of view it is a benefit.

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I do not recall if building factories in orbit was one of the "conventional" methods he looked at before describing this method, but to me that seems more realistic.

LB

True, they'd have to be using the trimmings. Just comparing the shell to the inside there's close to 99% waste. If you consider the outside it's probably 99.99% waste, assuming a 1 meter thick shell. It <b>has </b>to be the other way around. Although an Applied Statistics professor at San Jose State University once told us a story about a Russian Semiconductor Co. that made 100% waste for 2-3 years in a row without shutting down operations. Everyday, for 3 years, everybody came to work, ran a complicated set of process steps to produce semiconductors, and 100% of them were bad all the time, because there were impurities in the water. Apparently, nobody had the authority to tell anyone to stop.[] So, who knows, maybe for the first couple of trials, we're splattering iron all over the solar system.

rd

Larry,

A column of air 1" square starting at sea level and extending to an altitude of 250,000' or so weighs only 14lbs. In order for the "iron vaccuum balloon" to float it must be less dense than this, much less actually. Doing a rough guestimate I would imagine the thickness of the 1km balloon wall would need to be less than 100 cm, not 1m. I'd be willing to bet that the difference in density between iron and Earth's atmosphere is large enough that no "iron vaccuum balloon" could ever be constructed that could maintain its structural integrity at sea level. Maybe I'll calculate some shell diameters and thicknesses tomorrow if anyone is interested.

JR

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Larry Burford</i>
<br />If the initial trajectory is correct it will do most of its slowing down at high altitude, before drag becomes large enough to distort it and cause collapse.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">There's the fallacy. Drag is doing the slowing. By increasing the area-to-mass ratio, you have increased drag. So distortion and collapse occur sooner than they would for a denser body.

The problem is that the body needs to lose only a few percent of its speed before its orbital perigee drops below the Earth's surface and the body is committed to its final plunge. The speed losses and angle of descent accelerate as the body drops. When a body has collided with enough air molecules to equal its own mass, its average speed is reduced by a factor of two. But as the speed is dropping, inelastic collisions are generating heat. So if the collisions are too frequent, vaporization quickly follows. One generally can't keep the heat from vaporizing the body before major slowing occurs. Heat shields are one way to remove the heat faster. But whatever the process is, it must happen quickly because the air density soon becomes the equivalent of a brick wall for speeds of km/s.

A body with the specified dimensions would not achieve significant buoyancy until rather low altitudes, long after it was too late. The fate I would predict would be a catastrophic burn-through of the leading surface at high altitude, followed by air rushing in to fill what would then be a sail, which would produce the 'hitting a brick wall' effect and a spectacular high-altitude explosion. -|Tom|-